1. Field of the Invention
The present invention relates to a node structure capable of supporting a mesh-type optical network, and more particularly, to an optical node capable of supporting a mesh-type optical network in a wavelength division multiplexing (WDM) optical transmission system.
2. Description of the Related Art
As demand for large transmission capacities has rapidly increased, a wavelength division multiplexing (WDM) optical transmission system has been developed. The WDM optical system is considered as a solution for satisfying this demand. In the WDM optical transmission technique, a plurality of wavelength channels are concurrently transmitted in an optical fiber. For example, when a wavelength channel has a bit rate of 10 Gb/s, and fifty wavelengths are concurrently transmitted, the transmission capacity is 500 Gb/s. Accordingly, WDM is convenient for high capacity transmission.
In an optical network which uses the WDM optical transmission technique, an adding or dropping operation for adding or dropping wavelength channels is required in the network node so as to increase the efficiency and flexibility of the network. In a fixed optical add-drop multiplexing (F-DADM) technique, the wavelength of an adding or dropping operation of a wavelength channel is determined by the fixed wavelength filters in a specific node.
However, a reconfigurable optical add-drop multiplexing (ROADM) technique is needed for efficiency of the optical network and economical use of network resources. When the ROADM technique is used, an adding or dropping operation of any channel can be performed at any node, and accordingly, the network resources can be more effectively used. In order to embody the ROADM technique, various optical switches have been developed. More particularly, a technique of a wavelength blocker (WB), a wavelength selective switch (WSS), or an integrated planar lightwave circuit switch (I-PLC switch) has been popular in the markets. On the other hand, existing optical networks mainly have simple topologies, such as a point-to-point topology, a single ring topology or a bidirectional ring topology.
FIG. 1 is a schematic diagram illustrating a ring type bidirectional optical network. FIG. 2 is a schematic diagram illustrating an optical network having a shape obtained by combining two ring type bidirectional optical networks of FIG. 1 with each other.
Referring to FIG. 1, four nodes including a node 101 are connected to one another through an optical fiber 103. Each node includes two inputs and two outputs. An optical transmission signal starts from a specific node and proceeds in a direction 104 or 105 through the optical fiber 103. In the optical transmission signal, a plurality of wavelength channels is combined using a WDM technique and passed through an optical fiber. A reference numeral 102 represents an adding or dropping operation of wavelength channels at each node.
Referring to FIG. 2, a node 201, which connects two ring networks, includes four inputs and four outputs. Therefore, an optical signal, which proceeds in the left ring network, can shift to the right ring network. The optical signal can also shift in the opposite manner.
FIG. 3 illustrates an existing node structure used for the bidirectional ring type optical network of FIG. 1.
Referring to FIG. 3, some of the wavelength channels dropped in a ROADM 1 directly are connected to an external client interface through a transceiver 302 so as to process signals having units of a wavelength channel. Some other wavelength channels are connected to the external client interface from an electrical cross connect switch 305 through a transceiver 304 so as to process signals being less than the wavelength channel.
On the contrary, signals transmitted from an electrical cross connect switch 305 may be converted into an optical signal in the transceiver 303 and added to the ROADM 1. Signals transmitted from the client interface may be added to the ROADM 1 through the transceiver 301.
The wavelength channels other than the added/dropped wavelength channels pass through the ROADM 1 as the optical signal. An operation of a ROADM 2 is basically the same as that of the ROADM 1 except that the signal direction of the ROADM 2 is opposite to that of the ROADM 1.
In the electrical cross connect switch 305, signals with data rates less than the wavelength channel are aggregated and/or groomed. That is, in order to transmit signals with data rates less than the wavelength channel to an external node, the electrical cross connect switch 305 changes the capacities of the signals into capacities that are suitable for transmitting the signals through the wavelength channel.
In the generally used node structure of the bidirectional ring type optical network of FIG. 3, the wavelength of the transceiver 301 is different from that of the transceiver 303. Specifically, the wavelength channel passing through the electrical cross connect switch and the wavelength channel which accesses the client interface without passing through the electrical cross connect switch have to be previously determined. When the optical network has a small scale, the demand for the wavelength channels can be estimated. However, when the optical network has a large scale, and traffic increases, it is impossible to distinguish the wavelength channels in the aforementioned manner. In addition, in said node structure, since the transceivers 301 and 302 have to have the same WDM wavelength channel, and the transceivers 303 and 304 have to have the same WDM wavelength channel to support bidirectional flow, two transceivers having the same wavelength are needed.